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Why does the argument for Newton's 3rd law for lift of wings still exist?
I had recently seen a video which explained the various explanations for lift and one of them mentioned about the newtons 3rd law. It stated that the airfoil shape causes the airflow to go down causing the plane to up but that does not work for the planes which have symmetrical airfoils.
This principal was also mentioned by this answer somewhere else.
Then, why isn't this principle ruled out for the possible reason?

You already linked an answer that literally says "Lift can be explained in several, equivalent ways", none of which contradict Newton's third law. So what's your question?

@Bianfable yes they do not contradict the Newton's 3rd law but my question was how would it would it provide lift in the case of symmetrical airfoil.

@Bianfable I do know about the 3rd law. The question is not about its violation of it. When the air passes from the upper side, it gets pushed down by inertia which causes an opposite reaction pushing the plane up. But, the same thing happens from the bottom side of the wing pushing the air upwards. Don't these 2 effect cancel each other in the case of symmetrical airfoil?

@SambhavKhandelwal At zero angle of attack the effects cancel out at there is no lift produced. A non-zero angle of attack breaks the symmetry.

Why shouldn't a symmetrical airfoil produce lift? If it's at an angle in respect to the freestream then it does produce lift pushing air downward. Note that even a not-symmetrical airfoil also has an angle at which it produces 0 lift

Do you have the link to that sh... ehm, video?

It's easier to visualize mentally by thinking about a helicopter rotor. Its blades are just wings going in a circle. The blades are symmetrical on older helicopters, but they still produce substantial lift, in the form of downwash, because although symmetrical they are still moving through the air at an angle and inducing a package of air above and below the blade to accelerate down. Modern helicopters used cambered blade airfoils which are more efficient because composite blades are torsionally stiff enough to absorb the twisting moment from the higher pitching moment of a cambered airfoil.

You seem to be suggesting that some random u-tubers possibly misunderstood interpretation of aerodynamic lift is sufficient to render one of the most fundamental laws of physics to be either useless or irrelevant. The cool thing about physics is you can test ideas yourself.  Next time you are in a car, stick your hand out the window with your fingers pointed into the wind. No lift. Point them a little up and then a little down. Feel the force? Now cup your fingers a little to make your hand shaped like an aerofoil and repeat the exercise. Then you can go back a leave a comment on that video.

Yeah, I have experienced that @PaulSmith. Thanks for helping me find a connection with everyday life

Lift is produced by two effects: Camber and angle of attack.
Symmetrical airfoils do without camber and rely purely on angle of attack.
How this works is explained here.

So symmetrical airfoils can't fly straight and have to be pitched up a little?

@AadirajAnil Yes, If you want to create lift!

"Lift is produced by two effects: Camber and angle of attack" I do not agree: camber only shifts the AoA for 0 lift but it doesn't contribute to lift, only AoA strictly contributes to lift.

@sophit That is a matter of perspective. A cambered airfoil at the same geometric AoA will produce more lift compared to a symmetrical airfoil. Only when you count AoA from the individual zero-lift angle will the lift be about equal. Still, lift distribution over chord and maximum lift will differ.

This answer is correct but not complete. Lift is the vertical component of displaced medium (air) that is greater than gravity. Camber and AoA are important factors in working out the resulting force, but so are density, velocity, viscosity, compressibility and area.

@PaulSmith Thank you for mentioning this. I was until now completely unaware of these important facts.

I would argue that the airfoil meets the relative wind with either a positive or negative angle of attack, causing a pressure (force) differential between its two sides. That pressure differential will result in a third-law force in the opposite direction.

Okay, but then the plane can't maintain a level flight

@SambhavKhandelwal Why? Note that an airplane in general doesn't fly exactly where the nose is pointing. It's perfectly fine to be pitched up somewhat while flying level.

Hmm but that increases the overall front area exposed on the front of the plane which increases drag!

Everybody has own mental model, and we like to simplify things, so sometime it works, sometime not. With your symmetrical air-foil you may notice that some explanation doesn't work well on such case.
The main problem: airplanes are complex, and Newton's motion laws works well with simple bodies, but with more complex cases... you should be careful. Is centre of mass/lift/... moving? What about air? We cannot assume it is homogeneous and without much disturbance (and air has much reaction).
But we cannot do a full FME simulation or Navier-Stock solution. Airbus and Boing will approximate, and with huge computational cost (and still: they will tests to validate models), not something we can do mentally to check how things will change when flying. So we need good but simple mental model (and possibly knowing the limits).
And sometime the problem is about people not strong on physics but with good experience flying (or just confirmation bias): when they want to explain things, they may accept wrong model, if it gives the same results as in real world.
PS: physics has never absolute statements. Underlying physics there are always assumptions and limitation we tend to forget. (Just remember the Gravity laws (of Newton): it was well know the problem of infinity, nobody take the laws as absolute truth), we may tend to forget it.


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Is an engineering degree necessary to become a pilot after CPL?
Recently, I was wondering if I could directly go to an aviation school after I'm 18 to pursue my career as a pilot. But, asking some people I was told that I would need some sort of an engineering degree or there would be little to no growth in my career.
So, I have the following questions:

Will having a degree help me get a better career and a good growth?
Will not having a degree affect me/ my career?
Do airlines give priorities to pilots with degrees?

in which country? for which company? I never heard of such requirement, but I also don't know every single legislation/company policy

@Federico In India and for any airline eg Indigo. Does it impact us? My friends told that we may just be flight engineers or stuff like that without a degree and wont be actually flying planes... I did check sources but im not sure

A degree would definitely help you become an engineer

A college degree or specifically an engineering one is not a legal requirement to become an airline pilot in any country that I know of. For military pilots, it's required, since pilot billets are normally commissioned officers, and officers in most countries need a college degree, with some exceptions. Test pilots undergo education that covers key parts of an AE degree.
Modern airliners are extremely complex, filled with dozens of distinct systems with different redundancy implementations. Most of the pilot's job is managing these systems, not just the flight controls. An engineering degree, especially one in aerospace engineering, can be very helpful in diagnosing and resolving issues with the plane.
According to statistics, only 4% of airline pilots don't have at least an Associate's degree:

It's legal to hire pilots without a degree. But if there is any competition for pilot positions, an engineering degree is a clear advantage. From what I've heard, in India it's a de-facto requirement - there's enough candidates with a degree to skip over ones without it. Furthermore, for India, it's recommended to have a degree specifically in aerospace engineering, aviation, or aviation management. Out of these, aerospace engineering is by far the most relevant to a flying job, as it specifically deals with system design and behavior.
In other countries, where there's a shortage of pilots or candidates, a degree isn't quite as obligatory. AE is still worth pursuing early on if you have the means, as it's a high-effort degree that will be very hard to fit into an airline's schedule later, and it carries more cachet than just any degree.

Does that mean that a Bsc.Aviation is ok? Because after seeing what we are thought in it, it does not seem very useful...

@SambhavKhandelwal I'm not sure. In the West, aerospace engineering is normally the best aviation-related degree - but I'm not closely familiar with this particular degree. B.Sc in aviation appears to be a generic and somewhat watered-down mix of engineering, flying, and business knowledge.

In my experience, test pilots seldom have aerospace engineeering degrees.  Attending test pilot school is considered a graduate degree (like a Masters) by the military.  There is lots of aerospace engineering required (and an AE background would be a strong benefit), but your 'average' TPS class has many non-AE members.  In general, the military seldom matches degree area with job function for uniformed officers.  They favor breadth over depth.  When they need career technical experts (that do one thing for 25-30 years), they use civilians.

@RobMcDonald Hmm, perhaps you're right, I've been going off a small sample of a few names and not the real thing.

@Therac No worries -- it is a small detail.  Certainly to be a military pilot (except an Army Warrant Officer) you need a college degree (to be a commissioned officer).  Past that, options are pretty open.  In the civilian world, there is no such restriction, but a college degree will help.  An engineering degree is not required to be an airline pilot.

@RobMcDonald Thanks for the further clarification. But, as mentioned by Therac, India has a lot of competition in this sector and so having an engineering will definitely help me out.

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What are these devices on or adjacent to an airport taxi-way?
Can someone identify and provide some description of the objects marked in this photograph?

Especially those marked with yellow and red seem to be placed in the middle of a road and that seems to be making the road unusable. How can a plane or airport car go that way, if there are some devices placed in the middle? What am I missing?

You can find answers to your questions in chapters 2 and 3 of the Aeronautical Information Manual. Chapter 2 covers lighting and visual aids. Chapter 3 covers airport markings.

The double yellow lines indicate the edge of the taxi way.  The lights you see are on the shoulder.

It is not a road that you are looking at, it is the edge of the runway / taxiway.
The double yellow lines are used to define the taxiway
edge from the shoulder or some other abutting paved
surface not intended for use by aircraft.
The blue lights in the yellow boxes are taxiway edge lights.
The poles next to the lights are blue reflective markers that will mark the taxiway in case the light fails. Systems for light control often work for groups of lights (e.g. sections of taxiways) or, if individually controlled, through a control signal on the power line powering the light. In the latter case, there is a small piece of electronics in each light housing controlling the intensity of that individual light and reporting back if the light fails.
The yellow objects in the blue boxes are signs. They indicate names of taxiways, runways, holding points etc. You see the backside of light casings, not the signs themselves. A bit further to the right you can see similar signs.
I am not sure what the item in the red box is.

A small side question -- if you please -- any idea why blue lights in the yellow boxes are each equipped with something that looks like an antenna? Remotely controlling each of light separately?

The poles next to the lights are blue reflective marker that will mark the taxiway in case the light will fail. Systems for light control often work for groups of lights (e.g. sections of taxiways) or, if individually controlled, through a control signal on the power line powering the light. In the latter case there is a small piece of electronics in each light housing controlling the intensity of that individual light and reporting back if the light fails. I don't know of any wireless system that use antennas for each light

The fronts of the signs can be seen to the right of the picture, above the plane's left wing (right wing from our viewing perspective).

The item in the red box looks like a plastic stick that has reflective stickers on it which identify the taxiway edge even when there are several inches of snow (enough to bury the taxiway lights). Low-tech but it seems to work, and you can observe similar devices at many northern tier airports. Others can be seen near the blue boxes, slightly lower in the picture.

These are things Florida-based pilots can only see on the Internet. Thank you.

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Why is there really only one basic design for passenger airplanes?
So when I look around on the airport, I don't really see many differently shaped airplanes.
They all look pretty much the same, only subtle differences.
 Image: Aircraft Recognition
Why is this design so common? Why don't we have more extravagant designs?
Or am I not looking closely enough to appreciate the differences?
It seems like there could be so much potential for a variety of designs, offering more space and comfort for passengers for example... Or is it too risky to try something new?

Every aircraft design is a compromise among competing needs. The short answer is that sameness you see is the result of the typical design being the best compromise. There is progress, however. For example, the image you show is out of date. Turbo-jet engines are no longer used. Turbo-fan engines are more efficient. Also, we now use 2-engine aircraft for routes where we once relied on 4-engine aircraft. Also, instead of everything being aluminum, extensive use is now made of composites. In the cockpit, it's now all EFIS.

Sorry, I just picked the next best "diagram" style picture of an airplane, that I could find, that looks close to what I'm referring to.

Wow you guys, these are excellent answers! Gonna be hard to pick my favorite!!

Parthole? Is that where the repairmen load the new parts into? Or maybe it refers to the bald spot over which some men part their hair?

I just found this video related to the topic... "like a drawing by an 8 year old minus all the guns" :D

Minimizing air resistance while considering cost and cargo. The same thing happened when air resistance became important for cars.  Especially European cars look very similar because there is a large incentive to have good fuel economics.

For most transport aircraft, aerodynamic efficiency is the key parameter as it allows lower fuel consumption. The layouts you see most often are the most efficient known:

For jets low swept wings with engines mounted under and ahead of the wings and conventional tail is most efficient known layout. Tail-mounted engines used to be common on smaller jets, but as that puts lot of bulk far aft, they are worse with respect to Whitcomb area rule, worse for weight and balance, and wing-mounted engines also help damp flutter. So new regiojets are switching to engines under wing too.
Propeller aircraft fly slower, so they have straight wings. Low mounted wings are slightly more efficient, but then the plane has to have long gear to maintain sufficient ground clearance for the propellers, so high wings are more common. T-tail is then simply used to put the elevator above the most turbulent wake of wing and engines.

The only deviations from these three basic layouts are on special-purpose plane that have other more important concern. The most notable is military cargo planes are generally high-wing so they can sit low on the ground for easy loading and unloading using built-in ramp. Their marked anhedral is to avoid overstabilizing the aircraft in roll as both high wing and wing sweep increase roll stability. Earlier regiojet designs used tail-mounted engines for the same reason; to sit lower on the ground so they can be easily loaded via built-in airstairs and these are not too heavy and bulky.

I thought the t-tail is for elevator and rudder authority at high AOA,like takeoff and landing?

@rbp: The factor is speed more than AOA. At high AOA the air is deflected more, so the wake does not move that much. But at low speed the lift on the elevator decreases and putting it out of the turbulence helps to maintain control authority.

Oh i meant that the wings block airflow over a straight tail empennage at high AOA

@rbp: I understood that. But no, they don't (unless I seriously misunderstand the physics). The wake stays with the aerodynamic axis of the wing (so approximately the same place relative to the aircraft) as long as it is not stalled. And when it is stalled, it usually gets above the elevator in low tail designs, but hits the elevator in T-tails producing difficult-to-recover “deep stall”.

In fact, a high-wing aircraft is aerodynamically more efficient due to positive wing-body interference. Its main problems are structural (transferring the load from landing gear to the wing through thin curved fuselage) and fuel delivery (need to pump the fuel up from anhedral wing to the engines, considering possible failures and emergencies). Lower fuselage position is technically an advantage, but the airports infrastructure is already built for low-wing aircraft. It is also claimed that low-wing may be safer for emergency landings, but this is a moot point.

"For jets low swept wings with engines mounted under and ahead of the wings and conventional tail is most efficient known layout" - Why does it make a difference if the engines are above or below the wings?

@thosphor, because pylon on which the heavy object is hanging requires a bit less stiffness than one on which the heavy object is standing (and for cabin noise).

The aircraft industry did the bulk of its research in the 1920s and 1940s. There were literally dozens of variations of airframe and wings. After some experimenting and theorizing (at NACA, Farnborough and elsewhere), for each design goal/market niche there emerged one dominant design that fit the payload to fuel and other constraints and minimized cost.
Pretty much in passenger airplanes was decided by the DC-3 design, and in the jet era, by the Comet.

EDIT: Jan Hudec points out two significant improvements that the Comet did not have:

anti-shock bodies, first introduced in this design;
moving engines forward and into nacelles.

As an aside, there's another widespread addition to designs - winglets.
References:

http://people.bu.edu/fsuarez/Fernando_Suarez_Website/Publications_files/1995_SMJ_Dominant%20Design%20Survival_Suarez_Utterback.pdf
Bernie MacIsaac and Roy Langton. (2011) Gas Turbine Propulsion Systems, John Wiley & Sons.

There was significant change to the preferred design after comet when shock bodies and engines under and forward of wings were introduced.

well, and round windows

I looked at the 1945 study and thought, why did they show the top view pointing backwards relative to the side view, and then realized...oh!

If you look around a reasonably active hub airport, you will actually see many different design choices for passenger aircraft, all being produced today:

Some have high wings (above the windows), some have low.
Some have swept wings, some have straight.
Some have engines mounted below the wings, others have them attached to the fuselage.
Some have turbofan engines, others have propellers.
Some have conventional tailplanes, others have T-tails.
Some have two engines, some have four.
Most have a long roughly cylindrical fuselage; some have other shapes (consider the 747 whose fuselage cross-section changes dramatically halfway down the plane).

If you look at planes optimized for flying similar distances with similar numbers of passengers (compare, for example, Airbus A320 with Boeing 737) they will have the same choices in most of these categories. That's because the choices do make a difference, and for that particular use a particular combination turns out to be economically advantageous.
However, once you look at planes with different roles, things change. A 70-seater regional such as the ATR 72 does not look like a scaled-down A320 at all.
(Except very coarsely at the level of "long narrow fuselage, one pair of main wings near its center, a group of stabilizers all the way aft, tricycle gear").

Among jet aircraft, all new designs are now low wing with wing-mounted engines and conventional tail. Even new regiojets (E195, C-series, Su-100) that used to use tail-mounted engines in earlier designs. The do look exactly like scaled-down A320.

Propeller aircraft obviously won't look like jets, but among them the design variation is low too. Somewhat more common is high straight wing and T-tail (ATR42/72, Dash-8, F27), less common (but more common on smaller ones) is low straight wing with engines mounted on top of the wing and conventional tail (Saab 2000, metroliner, B1900D).

@JanHudec - The 3 types you mentioned aren't really old-school Regional Jets, they're competitors for the 737/A320. Actual smaller regional and business jets still usually use tail mounted engines.

Every time a new aircraft is introduced, the marketing department of the respective manufacturer claims that now a new era of air travel has begun. Remember the claims made at the time when Boeing launched the 747: We would enjoy a big-screen cinema and a bar in the sky, and much the same (plus the option of a flying spa) happened with the A-380.
In the end, customers vote with their wallets, and airlines need to earn money, so all those extravagant options wither and everyone comes back to what works best. Aircraft design has matured, and all those claims of blended wing-body configurations will only keep marketing and the press occupied.
Add to this the constraints of existing infrastructure and excessive regulation (thanks to which we enjoy unparalleled safety in air travel, btw.), and the potential advantage of new configurations will quickly vanish. Just try to find a way to quickly evacuate one of these blended wing-body configurations with 20 seats in a row, and any claimed aerodynamic advantage (which it doesn't have) will be moot.

It's called "convergent evolution". If you have one task to perform, and that task has physical requirements, all designs will eventually look and work alike. There are many side-trips along the way and those side-trips tend to stay around servicing their corner of the species.
Examples:

Large aircraft are low wing. Exceptions are heavy cargo and short / rough field props like the Dash-8 (big props don't fit on low wings) ( a 747 is not a heavy cargo aircraft. each piece is small)
Everyone puts their engines on the wings. The ones that don't sit very low to the ground and don't have enough clearance. Next best place: the tail. Good luck hanging a Trent 900 on either side of the tail. And the elevator has to move out of the way. The number of engines you have is purely a function of needed vs. available thrust. Notice how the trijet configuration went extinct as more powerful (and reliable) engines evolved.
all pressurized aircraft have circular cross sections and rounded windows, easier to inflate. Non-pressurized planes still use lots of flat panels.
fish-type creatures use a single large fin for propulsion. True fish wiggle it side-to-side, aquatic mammals that returned to the sea wiggle it up and down, like the legs they used to have.

Things like winglets, canards and other protuberances are bolt-on fixes for aerodynamic issues discovered after the initial design was completed. They likely won't be there the next time a blank-page design is started.

I disagree on winglets or canards being bolt-on fixes. While winglets may be added to exisiting designs, pretty much every design you'll see nowadays will have winglets from day one. And canards are such a radical and crucial design feature, that I doubt one even can "bolt them on".

Your example of aquatic creatures using tailfins to swim is rather poor, since it quite clearly shows that not all physical tasks constrain the solution space enough to converge on exactly one identical solution: horizontal vs. vertical fin is a pretty significant difference.

@NathanTuggy 1. Don't go hunting around for old discussions to comment on. 2. My point is that the aquatic creatures have a single big fin at the back. Up/down vs left/right is a very small difference compared to two or more opposing fins, jet propulsion (like cephalopods) or something else that evolution discarded and we don't know about.

@paul: This isn't a discussion; this is a Q&A site, and therefore meaningful critiques are always timely. (No hunting involved.) And I still think you're downplaying the differences too much; horizontal vs vertical is like V-tail vs horizontal+vertical stabilizers.

@NathanTuggy V-tail, T-tail, conventional and inverted-V all do basically the same job from the back of the aircraft. An elevator/rudder in front would be different. And Olli's question appears to be well answered.

The design is popular because it happens to be the best so far for subsonic large-capacity air travel. Other designs would either be less safe or have more drag, plus, there is not reason to fix what is not broken.
Boeing has flirted with different airliner designs in the past few years, but it seems that for the time being the most extravagant designs we will have will be the Boeing 787 and Airbus A380.

And not that while 787 introduces a lot of new technologies, aerodynamically it is not very different either.

@JanHudec - after the dominant design settles down, folks start toying with technology. Hence, composite airframe, lithium batteries etc. etc.

@DeerHunter: Yes. There are many new things on the new designs. But the layout is the same because that part is well understood already.

It would be a risk to try something new.  The airframe builder's goal is to make money and they can do that by improving efficiency vs the previous generation and occupying slightly different niches in the market compared to the competition.
What are the alternatives to the tube and low mounted swept wing?

High wing.  This often necessitates a T-tail which has inferior safety.  With the low wing the wing and landing gear can be mounted to strong points of the airframe.
Mid wing.  Fighters use this because it's more efficient but it would reduce cargo volume on an airliner.
Blended Wing and Body (BWB).  This design has many passengers away from windows and presents evacuation problems.  It's also easier to pressurize a sphere or cylinder than the BWB shape.
Delta wing.  http://seattletimes.com/html/businesstechnology/2002973147_boeingconcepts05.html

The Boeing Honeydew concept uses a delta wing.  Evidently the low speed handling/safety issue is considered too big a problem.

Canard.  The Boeing Kermit Kruiser.  The plane would get lift from the canard instead of downforce but IIRC the area ruling is more difficult.

I have seen others answer the question of why this shape (aerodynamic efficiency) but not why it is aerodynamically efficient.
The key in producing an aircraft with high Aerodynamic efficiency(lowest drag) is lowest possible wetted area and frontal area while simultaneously generating maximum lift.  Frontal area is just what you think, the lowest possible cross section that you try to push through the air.  Whetted area however is the the area that gets "wet" ie the total surface area of the aircraft.  This means that the best possible shape is a long cigar:  the thinner the better.  Any area that you add to the "tube" is wasted energy.
Simultaneously the wing shape that generates maximum lift with lowest drag is a long thin shape.  This is because wings cause lift primarily by creating negative air pressure by making air flow faster over the top than the bottom.  This effect is destroyed by having a wide, fat wing because the wing can't create the same tight pocket of air differential between top and bottom that a thin wing can.  This is what kills Delta wings and lifting bodies.
http://www.discoverhover.org/infoinstructors/guide8.htm
http://en.wikipedia.org/wiki/Lift_(force)
Once the speed of the aircraft goes to a larger percentage of the speed of sound however other factors come into play.  The wing must be swept and the wing must be stronger to handle the forces in play so it must be thicker.
The last portion is the tail in back.  This is just mimicking the arrow: putting the fletching in the back of the arrow creates natural stability by making sure the front of the aircraft stays pointed forward.
There are many advantages to other designs, Canards have great stall behavior, lifting bodies have superb strength and therefore safety and tandem wings with struts have great strength and great lift characteristics.  But the airline business is a high volume low margin business so any tiny bit of drag gets voted down when decisions get made.

Minimizing the drag for a given lift in a design is not the key for airplane. Minimization of operating cost is the key. You can have the best wing with a high aerodynamic efficiency but weighting several tons more...

Maybe something was lost in translation but this doesn't make any sense in english.  There is no Commercial airplane design scenario (which is the subject of the question) that fits the situation you describe (A high lift, low drag wing "weighing several tons more").  Also minimizing drag so profoundly minimizes operating costs that separating the two doesn't make sense.

As others have noted, the current general design of passenger aircraft is the result of decades of engineering and research. When all of the requirements are analyzed, this basic layout is the one that provides the best balance of advantages and disadvantages. The infrastructure has also been built around this configuration, which introduces another limitation that a new design would have to work around. While airports were willing to make changes in order to accommodate the A380, a completely different design would probably involve a more drastic change which would be harder to sell to airports.
Over the many years in which this design has evolved, many good lessons have been learned. Today's safety record in aviation is a result of these lessons being put into practice to improve the design. For a radical new design, much of this would have to be discarded or at least deeply reviewed. Research and testing would have to be done in many areas to ensure that the important aspects of the design are correctly understood.
Another key element is certification. In order for the FAA, EASA, etc., to grant a type certificate on an aircraft, there is a long certification process. For a completely different design, the certification would also be completely different. Not only would the designers have to be confident in the new configuration, but they would also have to convince these authorities. This would involve introducing a lot of new information. Recent new aircraft have already been taking longer and longer to certify. Introducing a completely new configuration could take even longer.

From the 30s to the 60s, the standard for transoceanic travel was a 4-engine aircraft. If you'd asked certain airlines if they'd buy a 3-engine Lockheed or Douglas airliner, they'd say "NO" because their customers would refuse to fly with less than four engines!
But gradually 3 engines became standard for oceanic travel.
And in the early 80s, ETOPS came along because engines had become extremely reliable.
So the design process is evolutionary; most changes happen pretty slowly, and designers often copy the successes of other designers.

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Helicopter aerodynamics
The formula in Prouty's book shows

the blade inertia unit is slung-feet squared
rotor kinetic energy unit is foot-pounds

if blade's inertia is high, the rotor kinetic energy should be high.
In other words, blade inertia and rotor kinetic energy are two different terms for the same thing.
am I right?

The term "inertia" is used to mean several different things: mass, momentum and moment of inertia. Without more context, I can't tell which meaning is intended in the formula you don't quote. However, none of these things is the same as the kinetic energy, as you can see immediately from the different units.

How is it unclear what he was asking? Emil answered pretty accurately.

Blade inertia and rotor kinetic energy are NOT the same thing, but they are related.
Blade inertia is a characteristic of the blade. It is a quantity that shows how difficult it is to change the blade speed (either by acceleration or deceleration) The bigger the inertia of the blade, the harder it is to make it move or make it stop once it's moving. It has nothing to do with the speed/RPM of the blade, it has the same value regardless if the rotor is turning or not. It depends on the way the blade is built (how heavy it is and how is its weight distributed).
Rotor kinetic energy depends on inertia AND rotor speed (RPM). So, a rotor when it's stopped has inertia but has no kinetic energy.
Once you start turning the rotor you start building kinetic energy in it using power from the engine. If rotor inertia is big, the engine will work harder to accelerate the rotor BUT, if the engine fails in flight the bigger inertia will oppose RPM decay giving the pilot more time to perform the proper actions.


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Why do control towers have tilted glass panels?
Almost all control towers I've seen have tilted glass panels:

Amsterdam-Schiphol tower (EHAM), source.

What is the reason behind this choice?
How is the tilt angle determined?

Related https://physics.stackexchange.com/questions/303815/why-are-the-windows-of-bridges-of-ships-always-inclined :)

I do not think this should be moved, anymore than other aviation related questions that have simple physics answers, like why fighter jet windows have metal in them or why runways use asphalt instead of cement.  My criteria for determining if it belongs on this site is usually "would people here be interested in it" not "would another, possibly nonexistent SE be a better fit"

If anyone comes up with an official answer (such as from the FAA) on why these windows are slanted, could you please add that information to the Physics.SE question Stelios linked?  I'm the creator of the "accepted" answer on that.  I later felt that CandiedOrange's answer was better, so I'm still curious if there's any sources.  An official statement might help clear that up... neither of us could find a citable source.

@CortAmmon Per this, refer to “Study of reduction of glare, reflection, heat and noise transfer in air traffic control tower
cab glass” by J. Michael Clinch

@bishop Thank you for finding that!  That answer has been a thorn in my side for a while!

@bishop: Very good find. This document refers to FAA-RD-72-65. In this document, 4.1.2 Reflection Control--Glass Tilt talk about a recommendation of 20° (while the study found only a 15° tllt in the towers the author visited)

@qq jkztd, I imagine the actual reasons have a lot more to do with aviation and aviation safety than they do architecture.

I'm pretty sure that has to do with reflections especially at night.
If you have ever stood between two mirriors you will understand the infinity mirror.

Light bounces back and forwards between the mirrors so when you look into them it appears like the scene goes on forever.
Window glass reflects a certain amount of light, like a mirror. In a control tower, where windows are all around you, if the surfaces of the windows are parallel you get that infinite mirror effect.
During daylight hours this can be seen as glare, but at night when you are trying to see that navigation light on Flight 594 amid a sea of runway and taxi-way lights, having your vision further confused by a myriad of reflections from the back window would be a bad thing. Further, since the room is a glass box, light can bounce all the way round the room. It would be quite possible to look off to your right and think so you see an unidentified aircraft approaching, but be actually seeing some truck's lights on the hill a couple of miles away over your right shoulder.
Tilting the windows outwards means the reflection angle is towards the ceiling, which is generally painted black, and the effect goes away.
Further, tilting out means you can no longer see your own reflection, or reflections from other lit workstations in the window. Ever tried looking out of a window at night into the dark and only see your own ugly mug staring back at you, then tried to move your head to the side to see around yourself. LOL
Of course, for all this to work, the ceiling should be dark.
A few other reasons include, less affected by rain, fewer refraction effects, and external reflections. Bonus feature, less greenhouse effect and AC bills. Downside... darn hard to clean.

inside the tower the reflection is towards the ceiling, and outside the tower it is towards the ground, which should prevent glare from the sun hitting approaching aircraft.

@Owen yes that may be an effect too, but since the aircraft are moving and the tower isn't the plane could still move into the reflection path. Even then though, one would think the reflections from the myriad of terminal windows would be a much greater problem. But still, you have a valid point.

Corroborated, though without attribution: http://www.livescience.com/32447-why-do-air-traffic-control-towers-have-slanted-windows.html

Avoiding opposing parallel windows could much easier be accomplished by using an odd number of windows (e.g. regular pentagonal or septagonal base area).

Related: Investigations of Characteristics of the Pentagonal Tower Cab

The (downward) tilted windows are common to ships as well. One of the reasons for its use is to avoid reflecting direct sunlight to the surrounding.

@Curd true, good point, but it is not just direct parallel, but bounce around. I expanded paragraph 5 to mention that too.

@Krumia Ya that is a popular explanation, though I think more of a myth, especially in the shipping world where the window is rarely vertical for long...

@Krumia, tilting the window down just moves the point to which sunlight is reflected closer, but does not remove it. However, the reason with internal reflections is applicable to ships as well—the crew often needs to look for a small light on the horizon in otherwise pitch black night, so they want to avoid reflections from lights in instruments on the bridge in their view.

@JanHudec Tilting the windows down prevents sunlight from being reflected outside a certain ground radius due to the sunlight being blocked when the sun is below the horizon. If the windows were tilted upwards, it would be possible for the sunlight to be reflected almost anywhere on the ground. The greatest radius is at sunrise/sunset, the least is roughly at noon.

As FAA (Order 6480.7D) used to put it: Positive tilt outward eliminates reflections from consoles and provides shading at high sun angles.
Another reason for tilting the Visual Control Room (VCR) windows is less deposits and less need for cleaning. This is particularly important to avoid rain impacting on the controller's ability to see from the tower.
In fact, many other typical planning and design requirements for control towers (and for airports in general) are based on visibility, such as the use of dark ceiling tiles in the VCR, restricting the use of solar farms in and around airports, or the highly controllable VCR room and desk lighting, among others.
The tilt angle is determined by the appropriate authority, which depends on the country but in the USA is the FAA. From my experience, tilt angle is usually set between 15 degrees and 20 degrees.
Another fun fact is that the increased sun shading provided by the window tilt helps keep the VCR cool. Keep in mind the VCR is essentially a greenhouse!

The glass is the most transparent and easiest to view through when it is strictly perpendicular to the line of view. Otherwise it scatters the passing light, adding "fog" that makes remote objects more difficult to see and identify. Following Lambert's cosine law, radiant intensity observed from an diffusely reflecting surface is directly proportional to the cosine of the angle θ between the direction of the incident light and the surface normal". At the right angle, cos(90) = 0, so. This should be much more noticeable when the glass is not perfectly clean (see also here).
If this assumption is true, the slanting angle should be such that the glass is perpendicular to the most usual line of sight into the ground from above.
As other answer suggests, avoidance of the direct reflection (this is is not a diffused scattering) may be another reason.
Railway control towers, where present, also have the similarly slanted windows.

I considered that too but dismissed it. Reasoning: Although a valid phenomenon, it would bias the windows for a downward view. and significantly impact the view of aircraft in the air. Since controllers must view both, such a bias would be counter productive. It would be better to bias somewhere in the middle.

@Trevor is correct. The most important direction for air traffic controllers to look is in the sky which is generally towards the horizon or slightly up. Using this logic the windows should be normal and not slanted. Thus this can't be the reason for slanted windows.

@slebetman.  I think you're making a very big assumption that ATC manage traffic in the air by visual means. Just maybe radar might be used more? Just how far can you see anyway? On the ground however, visual confirmation that a taxing aircraft hasn't crossed a runway it shouldn't have, is more likely. Often aircraft will be instructed to "hold short" of a runway until a landing aircraft has touched down and exited the runway.

@Penguin although that is likely true, it does not preclude the other.

Lambert is the correct primary reason, but another factor is the reduction of particulate adherence.

This is great reasoning for a railway control tower, but airplanes fly.

@Octopus The primary reason is still to keep dirt/rain away and so as to reflect any light originating from computer monitors upward onto the ceiling, reducing internally-generated visual disturbances at night. The windows would be mainly used to visualize aircraft on the ground; aircraft post-takeoff are (largely) monitored by electronic means. Human visual judgement of distance and velocity isn't precise enough alone, one aircraft leave the tarmac.

Machicolations ;-)
I'm not building control towers, but I think one reason would be to allow for better viewing of what happens on the ground near the tower.
It may not be needed so much with an airport control tower as a railroad control tower (and most are using screens anyway), but there is also the comfort for passengers that a control tower looks like what they expect: Up there there are people looking down on the airport and making sure we are all safe.
If the windows were vertical or slanted upwards, it would look "wrong", and signal that the people in the tower weren't interested in little me on the ground.

"there is also the comfort for passengers that a control tower looks like what they expect" sounds like Catch-22 logic to me.


text:
Is it very rare to be a senior pilot with no emergency occurrence in history?
I was reading a Civil Aviation Safety Inspector from Transport Canada:

I have met a few pilots who have flown all their lives without
  declaring an emergency. Pilots such as these are very rare.

Source

Emergency landing for this Boeing 727 flying into a serious hailstorm over Switzerland (source: ASN)
Is your experience in line with this TC inspector statement? Is it very rare to be a senior pilot with no emergency occurrence in history?
Let's define emergency as a mayday or pan-pan radio call or transponder code.

Comments are not for extended discussion; this conversation has been moved to chat.

In the airlines, the number of flights where an emergency has been declared is about 1 in 600. In general aviation, it could be as low as 1 in 40 or as high as 1 in 6,000, but it's not "most".
In the airlines:
Poking around Google gives this USA Today story: Medical emergencies occur on 1 of every 604 flights, which references an article in the New England Journal of Medicine.
An answer on a different question on this site provides the following anecdote:

had occasion about a year ago to ask our airline safety department
what the most common cause was for declared emergencies in our
operation, and was told that far & away, sick passengers (medical
emergencies) were #1 by a huge margin, and everything else (which
includes all the aircraft-related events) was far down in the weeds in
comparison.

(emphasis added)
We'll assume that Ralph J's answer is correct and that medical emergencies are the lion's share of total emergencies. That being the case, the 1 in 600 number is probably accurate within an order of magnitude.
In general aviation:
Since 2001, the NTSB aviation accident database has been remarkably consistent in terms of the number of reports they get:

About 4,000 reports per two-year span (until 2013, when it dropped to a little under 3,500)
Between 60-90 reports containing the phrase "declared an emergency" within that span

As airline travel continues to be outrageously, shockingly safe, let's run with the assumption that most accidents in the database are non-airline accidents. That being the case, one in 30-40 of accident airplanes (not all airplanes) declared an emergency in a way that managed to make it into the accident report.
That doesn't fit with a fact I found in an AOPA handout though, which says that air traffic controllers provide more than one flight assist per day.  "More than" one could be anything, but let's be super generous and say there are 1,000 flight assists per year. Another FAA source says that there were 24 million GA flight hours in 2015. Figure a GA aircraft holds about 4 hours of fuel, then there were around 6 million GA flights in 2015, or one flight assist per 6,000 flights. That throws a wrench into our other estimate, but it also goes against the "very rare" statement of the inspector in the original question.
From personal experience
Anecdotally, I've flown a few thousand hours and I would say that I've heard aircraft on the radio requesting emergency services less than a dozen times. If I had to guess, I would put the number closer to the 6,000 side than the 40 side, but even if 1 in 40 was correct, 39 pilots out of 40 not having declared an emergency seems to contradict the statement of the inspector.
Conclusions
The TC inspector could be suffering from selection bias. If she or he deals with emergencies all day, it seems less likely that she would come into contact with the (seeming) majority of pilots that haven't declared.

"1 out of every 600 airline flights have an emergency declared" and "1 out of every 600 airline pilots have declared an emergency" don't seem to match unless the average airline pilot makes one airline flight in their lifetime. Am I missing something?

@TannerSwett - Nope, you're not, I didn't catch that.  Darn

Thanks for the answer. Quick estimate: 1 emergency/600 airline flights x 100,000 flights/day x 365 days x 3 years ≈ 180,000 emergencies in 3 years. Let's say there are 180,000 airline pilots in the world ⇒ every airline pilot will have lived an emergency within these 3 years. Is it correct?

@mins - More like "In 3 years there will have been enough emergencies that there would be one for every airline pilot if they were handed out equally."  In practice I suppose it would work out to some sort of bell curve?  Also, bear in mind that this isn't rigorous analysis and I am entirely untrained in this subject.

If distributed uniformly, right. Another angle of extrapolating would be your experience: 12 times for the frequency you listened to, times the number of ATC facilities. I'm surprised by the high ratio, as for the birthday problem.

@SteveV. Might be worth editing the question to reflect that when you have some time?

"In general aviation, it could be as low as 1 in 40 or as high as 1 in 6,000" One in 40 is far more than one in 6,000. The former corresponds to about 2.5% or 150 per 6000, while the latter corresponds to about 0.017% or 1 per 6000.

@SteveV. Do you have any plans to edit this answer to account for the fact that airline pilots make more than one flight per lifetime? Perhaps you should change "1 in 600 airline pilots" to "most airline pilots".

If there's a 1 in 600 chance of an emergency on a given flight, then a pilot's chance of never having to declare an emergency in $N$ flights is $(599/600)^N$.  After 2000 flights, this number is down to about 3.6%.  This calculation can be modified (in a hopefully obvious way) for whatever "emergency rate" and number of flights in a career you think is reasonable.

Reminder that this answer, which its author acknowledged has a fundamental mismatch with the question, has simply remained here for years without being corrected!

@nanoman - I encourage you to add your own answer, or you're welcome to edit mine with newer or better analysis!


text:
How does a commercial airliner measure its weight/mass?
Other questions currently being answered cover situations regarding planes near, at or even above their MTOW. It raises a more basic question: how is the exact "ramp weight" of an airliner measured or estimated?
On most airlines, luggage is weighed and counted, so the pilot would know within 100lbs or so how much mass is in his cargo hold. He'd also know the "operating empty weight" of the plane itself (the weight of the airframe, all fluids besides usable fuel and the weight of the crew), and the mass of the fuel he's taking on.
The biggest variable, then, is the weight of the passengers and their carryons/personal effects. Last time I flew, I wasn't knowingly weighed and neither was anyone else getting on with me. So, while you know the number of passengers, the total "cabin weight" is a big unknown.
So, do planes have load sensors in their gear which could be used to exactly measure the plane's ramp weight? Or is a median passenger and carryon weight used, with a more conservative total passenger rating used in case a few planeloads err on the heavy side of the bell curve?

You know how you have to pay a lot of money if your bag exceeds 25 kg, and how you have to buy two tickets if you're too big to fit comfortably in one seat? There's probably more than one reason for that...

Related: Do airlines weigh passengers and baggage before loading the aircraft?

@Landak - I figured that these things factored into the weight calculation. However, passengers aren't weighed directly while luggage is. On my last flight, my toddler daughter required a ticket for her own seat. She weighs 30 lbs soaking wet. I weigh... more, and I was carrying a second child on my lap. So obviously there's still a massive range of possible passenger weight per seat. It falls in a bell curve, like almost anything, but if you're ferrying a pro football team, that's a lot of lean muscle aboard and thus a relatively higher per-seat weight, so how might that be accounted for?

Related: Do some airplanes have weighing scales on the landing gear?

See https://en.wikipedia.org/wiki/Air_Midwest_Flight_5481, where total passenger/luggage weight and CofG errors contributed to the crash...

Such a great idea! All modern semi trucks have a gauge on the dash which simply shows the pressure required to stabilize the airbags (suspension) under the drive axles. From this, weight and balance can be estimated. And that’s just a semi truck! With a pressure sensor in each tire, I see no reason why aircraft could not achieve more precise w+b. Of course you’d need temp sensors to achieve on-the-spot calibration, but not much else.

Also, conveniently, what with the plane being airborne just before landing, it has this epic opportunity to get its ‘unladen’ tire pressures, plus it knows the atmospheric pressure, so I’m pretty sure this would be stupidly easy to engineer: get tire pressure, ambient pressure and temp before landing. Then (later); get tire pressure, ambient pressure and temp when you are reloaded and ready to W+B. Calculate differences.

Short answer
Determination of mass and balance (or weight and balance) is a critical task for the crew, an error in this operation may have tragic consequences.

Weighing passengers and carry-on items is mandatory per regulation.
Either standard predetermined weights, or actual weights can be used in this operation. Airlines usually use predetermined tables. Tables values take into account different types of travel (e.g. regular and charter).
While the weight must be checked, the center of gravity location is of equal importance. The distribution of the passengers in the aircraft is also taken into consideration.
Aircraft are not weighed prior to each flight, but the basic empty weight must be known and checked periodically per regulation.
Aircraft are weighed simply using scales under the carriages :-)

(source)
The other method is to use jacks. See further below for an image.

So this was the short answer. To understand why and how to balance the aircraft, there are quite a lot elements to introduce. For those interested, just continue!

Weight...
The different elements composing the takeoff weight:

All of these elements must be determined prior to taxi and takeoff.
The mass of these elements are necessary to check that the different maximum weights are not exceeded. However there is another aspect of the masses which needs to be secured: Where are the masses located?

Taking KeithS' example, if there are many heavy people grouped in the forward section, we may have a problem at Vr, even if the MTOW is not exceeded.

... and balance. Location of the center of gravity vs. the MAC
The masses must be distributed so that the center of gravity of the aircraft is located within limits determined relatively to the aerodynamic chord, and the center of lift.
This is required to ensure the stability and the maneuverability, to minimize the efforts on the horizontal and vertical stabilizers and to increase the maximum lift and reduce the drag. On airliners, the center of lift must be located between the CG and the center of lift of the horizontal stabilizer, so that the latter pushes downward to balance the weight.

On airliners the width of the wing is greatest where it meets the fuselage at the wing root and progressively decreases toward the tip. As a consequence, the chord also changes along the span of the wing. The average length of the chord is known as the mean aerodynamic chord (MAC).

The MAC is a portion of the wing, delimited by LeMAC (leading, 0%) and TeMAC (trailing, 100%). The CG must be located in the MAC.
In large aircraft, center of gravity limitations and the actual center of gravity are often expressed in terms of %MAC (on the roll axis, similar limits exist on the two other axis).

(image and text around: Aircraft Design, Mohammad H. Sadraey)
In general, in large transport aircraft, the most forward CG is located as forward as 5% MAC, while the most aft location as aft as 40% MAC. The average CG limits in large transport aircraft are between 20 and 30% MAC.
As commented by Calchas, using wrong weights for the passengers (adults instead of children) may move the CG to a wrong location, not balanced by the trim, and create strong difficulties during the takeoff and the initial climb.

PAX weight applicable procedures
For an airliner, the PAX portion is relatively small compared to the total mass, so the passengers mass is usually estimated based on standard passenger mass tables and the number of PAX per cabin section. Each airline may provide its own standard mass tables, but standard tables are proposed with the applicable regulation:

JAR-OPS1 or EU-OPS1 for Europe (section JAR - OPS 1.620 Mass values for passengers and baggage). Values valid in 2008:

(source)
Those tables have been establish using airlines data obtained by surveys. They are updated from time to time.
In case the operator wants to establish their own equivalent tables, JAR - OPS 1.620 provides a detailed operating mode to sample passengers and weigh them.

For the EU, a new survey was conducted by the European Aviation Safety Agency in 2009 and a proposal for table updates issued:

Passenger and Baggage weight survey

At this point you may want to download the documentation I'm mostly using for this post: Mass and Balance in Aircraft. We are at page 54.

Special cases
There are cases where it is obvious the actual mass and balance will be erroneous if the standard tables are used (this is especially true for a small cabin).
In these cases the operator must improve the evaluation by actual weighing of either all or a sample of passengers and their baggage. The procedure already mentioned for establishing standard tables is used (JAR - OPS 1.620).

Putting all elements together
All masses, arms (distance to the reference center) and moments are finally reported on the aircraft manifest to calculate the correction that must be used for the takeoff (trim amount).

Basic empty weight determination
The aircraft is not weighed for each takeoff, still it has been weighed to determine its basic empty weight and its center of gravity. This operation must be done every 3 or 4 years (9 or 18 years in a large fleet, this is a specific process) according to JAR Ops 1.605, Appendix 1.
Platform scales under the carriages may be used:

(source)
Another method is to use jacks equipped with a transducer at their top:

(source)

Related FAA regulations and JAR-OPS

FAA AC 120-27E and JAR-OPS 1.620 to determine the mass of the baggage, passengers and carry-on items.
FAA AC 120-27E, JAR-OPS 1.607, JAR-OPS 1.615 for the determination of operational items, which include the crew and the crew baggage (there is also a table, that I won't include to prevent comments on the differences with the passenger table -- All tables are based on survey from airlines). The operational items also include catering, emergency equipment, spare parts..

May be interesting too:

There is a confirmed trend of weight increase among the population (including myself). This has an impact on the operational costs. The way airlines manage this impact can be read in a two-part article: The true costs of heavier passengers, part 1, part 2.

Related:

Do some airplanes have weighing scales on the landing gear?
Do airlines weigh passengers and baggage before loading the aircraft?
How much time does it take to calculate the leveling and weighing of big and small aircraft?

Aren't those weights changed in recent years?

So what happens when there is an expected deviation from the mean/median weight per passenger? An American football team, plus coaching, training and support staff, would represent the bulk of the average airliner's passenger capacity, and would have a substantially higher per-passenger mass. How might the pilot account for the difference? The team would weigh their players regularly and so could furnish relatively recent specs on the players at least...

@KeithS Recently about 90 school kids on a QF 737 were incorrectly loaded as adults, and worse, sat at the back. The centre of gravity was therefore much further forward than the pilot expected. The pilot only noticed when he tried to rotate off the runway and found the nose very heavy; having to pull back so hard made him very nervous about a tail strike during the take off. The incident was investigated by the Australian Safety Board: http://www.atsb.gov.au/media/5092920/ao-2014-088_final.pdf

@mins I thought they were changed somewhere in the transition from JAR-OPS to EU-OPS to AIR-OPS, at least I remember seeing a proposal to change the standard passenger weights at one point. They were not, see this AMC (pdf) section CAT.POL.MAB.100. So disregard my previous comment. You've crafted a nice comprehensive answer, I like it!

The initial version of this answer was good; the updates with contingencies and total trip planning is excellent. Tick.

Thank you very much, @mins. Very complete explanation.

JARS-OPS in EU is now replaced by similar rules known as EU-OPS or IR-OPS or EASA AIR OPS or simply AIR-OPS. See this. So much names say a lot about the harmonization process... For mass and balance, see page 46 of this document.


text:
What liability does an aviation inspector have for errors & omissions?
In reading online, I've come across the term "pencil-whipping". The actual term is new to me but I'm familiar with the practice; it's when someone fills out the paperwork that attests something is true, without actually verifying it himself (at least not to the standards required). Technically that's fraud, but it's also usually a "who'll watch the watchmen" situation; if you don't know what the guy is required to do and watch him do it, or else call in a second opinion, you won't know anything's been skipped.
I purchased a home a few years ago, and the home inspector did a good job of finding the things we'd have to get fixed. If he hadn't though, State of Texas requires these inspectors to have "errors and omissions insurance" on top of a "general liability" policy. If the inspector said he has checked something though he didn't; or he made a mistake checking it; or he didn't check something he was supposed to, he and his insurer are liable for any damages resulting from the new homeowner trusting that report. This generally discourages pencil-whipping in the home buying industry, as after the first instance of the insurer having to pay out for an electrical fire caused by an outlet the inspector attested to be fine, the guy's likely out of a job.
The question here is, do A&Ps in the United States carry a similar liability? If I were to hire a guy and pay him thousands of dollars for a pre-purchase inspection of an aircraft, get the all-clear, buy it and then find out the aircraft actually isn't airworthy because of something he missed, what recourse do I personally have against the inspector? How would I report this misfeasance and to whom? Assuming nobody was hurt and I just have a loan payment on a museum piece (or a fat unexpected repair bill), what damages can I claim?

Please specify what laws that you are asking about.  I'm assuming US/FAA since you mention the state of Texas....

Minor comment on terminology: an ASI, Aviation Safety Inspector, is an FAA inspector, a "Fed."  His job is doing inspections for the FAA, not for buyers.  You're probably thinking of an A&P who is "inspection authorized" or IA -- that's more likely to be the guy you can hire to do the sort of inspection you're referring to. It's might be possible that somebody who happens to be an ASI could be willing to do an inspection like you're talking about, but he'd be doing it as an IA A&P, rather than as a Fed.

Edited to refer to the correct person.

There's a good article on this question for the US here (you mentioned Texas). The relevant FAR is 14 CFR 43.12:

§43.12   Maintenance records: Falsification, reproduction, or alteration
a) No person may make or cause to be made:
(1) Any fraudulent or intentionally false entry in any record or
  report that is required to be made, kept, or used to show compliance
  with any requirement under this part;

In other words, no pencil whipping. And if you do it and get caught, then the FAA can suspend or even revoke your A&P certificate.
But, that's only part of the story. The FARs say nothing about civil liability and state law. Even if a mechanic does everything correctly according to the FARs, someone can still sue him if they believe they have a civil case for negligence (just like suing a doctor or any other professional).
And that's where this question probably becomes unanswerable on this site: local (state) law will determine what - if any - liability the mechanic has. In the end it will come down to what's in your contract with the A&P, what your state laws on professional liability are, and what the specific circumstances of your case are.

I only can speak for Canada but we use much of the American regulations in Canada as part of our regulations. In Canada, all mission critical systems Ie flight control systems etc ... have to be signed off by 2 inspectors for a total of 3 people working on the system. The maintenance engineer will sign of there own work and the inspector will sign off the inspection. A second inspector will sign off the secondary inspection. So typically if there is an error one person is typically not solely responsible for it. Additionally all inspectors and maintenance engineers licenses are only valid while working for a maintenance organization AMO. The organisation's insurance typically covers the employees working for them. When I went through my licensing I was told that we could still be held liable for criminal prosecution if we were willfully negligent. It was explained to us this only incurred once previously.  If memory serves me correctly it was the mechanic responsible for a plane that went down in the Florida everglades.

Is this the case even for inspections and maintenance on light aircraft like Cessna 172s or whatever?

Yes it is. There is no difference in the inspection requirements.


text:
What's the point in escorting a threatened flight with two fighter jets?
Two days ago I read in the news:

Two U.S. fighter jets escorted a Canada-to-Panama flight back to
  Toronto after a passenger  allegedly threatened the plane Friday
  morning. The nature of the threat or why the passenger was agitated
  were not specified. CBS News  reported that the passenger told a
  flight attendant, "I have a bomb and I will blow up Canada."

What's the point of escorting the flight while it has been threatened by a passenger? How do they help to eliminate the threat?

My best guess would be that the second aircraft is there just in case the first one has some sort of emergency himself and has to abort the mission.

@Lnafziger fighters are trained to operate in pairs, watching each others' backs constantly. Just because there's no threat to the fighters doesn't mean you abandon SOP...

The current answers seem to be ignoring the psychological factors for the man with the bomb seeing the fighters, could potentially make him think twice if he has a fake one but they have very real missiles.

Fighter jets escorting planes after various sorts of emergencies seems to be standard procedure in many countries, you hear about it quite frequently. It's sometimes implied in the media that if the situation would turn into a 9/11-type hijacking it might be necessary to shoot the plane down but nobody seems willing to fully clarify who could take such a decision and under what conditions on the record.
Now, in most cases, there isn't even a suggestion that a hijacking is happening but it's difficult to determine that in a timely manner and there are still two things fighters can do in other cases:

Get a visual confirmation of the situation aboard the plane (Is the plane damaged? The cockpit windows obscured? Who is in the cockpit?)
“Guide” a pilot who has lost communication to an airport.

Also, one factor is that being able to scramble jets is often seen as a basic requirement to assert sovereignty (witness the mini-controversy in Switzerland when it was revealed that the air force could not do it at certain times of the day) so politically it seems difficult to entirely give up on it, even if it could be argued that it's a waste of money for smaller countries to maintain an air force that does very little beside this type of missions.

It may not be waste of money. It may be simply paid from the training budget and counted towards the hours the pilots would have to fly to maintain proficiency anyway. I don't know which countries do or don't do that.

In fact it is quite reasonable to count such mission towards training of the fighter pilots. They have to follow often quite complicated instructions from their controller (to avoid all other traffic), locate and identify the target and manoeuvre in vicinity of the target for some time, all things they would need on combat mission too.

@JanHudec That may be but none of this require 24/7 availability, which has a cost in itself. And if you give up on this mission, having fighter jets and training pilots on them might itself be seen as a waste of money for many countries.

@Relaxed pilots need a certain minimum amount of flying time to stay proficient (including a set number of night hours), combining those hours with operational missions saves on having them fly around in otherwise pointless circles. It also saves on airframe hours, ever more important with our fighter fleets aging to the point most of them are nearing their fatigue lives with no replacements budgeted for.

In the British case, a hijacked aircraft falls into the category of aircraft in (or near) British airspace without authority, and the same procedure is followed as for (e.g.) a Russian bomber approaching British airspace (this happens once or twice a month, in general). This is the only useful write-up of the British situation I'm aware of.

@jwenting I obviously know that so I am not sure why you would think it's relevant. The point is that it only saves money if you assume you need to have these planes and pilots (or even an airforce at all), which is the real question.

OK, but why 2 fighters? One should be enough to escort/destroy/help/guide an aircraft, even if having at least 2 available is understandable.

@ManuH Good question, possibly worth asking separately. Personally, I have no idea.

@SHAF yes, there are. But hours are important. So important in fact that many fighter pilots are now resorting to renting light aircraft to keep their hours up or they lose their license, the air forces no longer providing them enough stick time to stay current even on PPL level.

@Relaxed I think the question actually ask it ("with two figther jets")

@ManuH I don't think it does. If you look past the title and read the question itself and in particular the two sub-questions at the end, it's about two fighter jets as opposed to none at all, and not about two vs. one. Generally it does happen to be two, which is probably why the title mentions “two fighter jets” but that's it.

@SHAF explain that to the military pilots who do just that, rent aircraft to get more hours in because they feel they're not able to stay proficient on the 20 or so they get each year to fly in their F-16s...

@SHAF Note that jwenting is not in the US (and also frequently makes stuff up) so it's entirely possible that he is talking about another country or just spreading a rumor. In any case, I still don't see how this has anything to do with my answer so I invite both of you to move to the chat if you must continue the discussion.

@SHAF Yes, I understand that, but that's completely beside the point. As I explained earlier, what I meant is that it might not make sense for countries like Switzerland or Austria to have an air force at all (or at least fighter jets).

@Relaxed for smaller states having an air force is more of a political tool.  Being able to intercept foreign aircraft that beach their borders gives their state legitimacy on the global stage.  Even if they never fire a shot they can save face and claim to have escorted the threat away from their borders.

@SHAF Sure, I agree and I think that it's what I wrote in my answer, which is why I don't see the point of jwenting's remarks and of the whole discussion that ensued. Hopefully my recent edit can put this to rest.

@Relaxed Ah, I understand what you're saying now.  I'll simply delete my previous comments to declutter the comments

@Relaxed The only other thing I would add is that the political fallout from shooting down a passenger aircraft, without first getting a visual ID, no matter how justified, would be disastrous.  Not only would civilians protest, but foreign governments with citizens on board would be in uproar (think Ukraine).  Nobody wants to hit a passenger plane with a SAM.

Especially post-911 it became paramount to ensure that such aircraft form no threat to cities and other places where crashing an aircraft into them would cause serious damage.
Blowing it out of the sky, however bad it would be for the passengers (not to mention the psychological impact on the fighter crews and their controllers) would be preferable to having thousands of victims on the ground (and a major PR coup for some terrorist group).
So jet fighters are scrambled to escort the aircraft until it's on the ground somewhere or the threat otherwise neutralised (say the attempted hijacking foiled by people on board).

This is little different from fighters being scrambled to escort intruders into a nation's airspace (and in extreme cases shoot them down) which has been done since the end of WW2 and maybe sporadically before (without radar to detect intruders and guide interceptors it's a lot harder to do so obviously, and prior to WW2 that wasn't available).

In this specific case it may have been overreaction to a madman with a fake bomb, but not knowing whether the threat was real or not it's easier to send up the fighters and later recall them than to have to explain to congress and even worse the press why you didn't act after the jetliner crashes into some city center during lunch hour...
Be happy that they have the option to send up fighters and don't have to rely on guided missiles alone, as there's no recalling those once launched...

Don't they have guided missiles that they can recall?

@gerrit recalling a guided missile requires prematurely detonating them which may still damage the plane. and a pilot in the air can make better decisions after establishing visual contact

@gerrit recall? no. Destruct? Some models have that option, many do not (if you build it it can be jammed, and you'd prefer not to have your target order your missiles fired at it to destroy themselves). Of course with UAVs carrying warheads that can be ordered to patrol and return or sent on a suicide mission the line between aircraft and missile is getting blurred, but those are typically designed to take out ground targets, too slow to target aircraft.

"Infidel Americans shoot passenger plane out of sky. 300 dead" would also be a pretty good PR coup for the terrorists.

Of course, all of this presupposes that the hijackers actually manage to successfully gain control of the aircraft - not a good bet post-9/11.

@Sean With external help from ground it's still quite possible to get stuff into the cabin that doesn't belong there. Usually it's drugs (and cabin crew and ground handlers are regularly arrested for that), but weapons and explosives are certainly an option if someone is determined enough to infiltrate airport staff at the correct positions.

@jwenting: I wasn't referring to them getting weapons onto the plane (which is remarkably easy to do, even nowadays), but, rather, the great unlikelihood nowadays of the people on board passively allowing themselves to be hijacked.

In addition to @Relaxed's answer, one additional reason would be if the hijacker gets control of the cockpit and turns off the transponder. ATC works on secondary radar with a signal bounced back from the plane in order for ATC to track the aircraft.
If the transponder is turned off, it would be very difficult for ATC to track the exact whereabouts of the plane. By having escort planes, they could maintain positioning of the endangered aircraft.

The utility of sending fighter jets is to destroy the plane before an hypothetical attempt. It's unfortunate, but the point is to kill a few people to save many. This can also possibly have the impact of scaring the hijacker/terrorist.
In the mean time, someone in the ATC can attempt to negotiate with the terrorist before any shoot-downs are needed.
In France, we send two fighter jets, because one jet is there to make contact with the pilot (seated on the left of the cockpit) while the other fighter stays behind the aircraft ready fire if needed.
The only person allowed to make the decision to fire, is the Prime Minister.
In fact, fighter jets take off on these missions quite often, but it's usually only for providing assistance (e.g. providing landing clearance to an aircraft with dead radio).

In addition to the reasons above (911-ish & transponder out), it is also an indication that somebody on the ground is aware and cares if communications are cut off and passengers do not know what is going on.  They may not be able to do anything and passengers may realize that, but at least you know the ground is aware and you aren't alone.
As far as an on board nut-job being aware of any possible "trouble" he/she may get into, I think they usually don't care if they are 911 fodder.
When I heard that a second plane hit on 911, on the way to work, I knew life (mostly @ airports) had changed right then.


text:
How is a propeller diameter for given application determined?
let's say I need 1000 N of thrust for a given aircraft and flight velocity. I have available a fixed pitch propeller with a given shape, and thus given performance to advance ratio curves. Let's say the propeller diameter is 2m.
Now, what would determine if this is the best diameter for the application on hand? What if the blade shapes were scaled up to 4m? Wouldn't this generally be better for efficiency as the RPM would decrease and the air acceleration required would be lower? Other than practical factors like clearances, stability, etc., what aerodynamic reasons are there to stop increasing the diameter at a certain value?

Let's start over with the basics.
In any application, the prop tips have to be maintained subsonic or else propulsion efficiency will suffer and noise generation will become extreme. Therefore, for any given prop diameter, there is a do-not-exceed RPM value associated with the sonic limit: big props have to turn slower, smaller ones can turn faster.
(Prop diameters are also limited by ground clearance considerations but for small planes like a Cessna 172 or a Cherokee 160, the need to keep the prop tips out of the grass can be accomodated readily.)
Next you have to match the prop with the best power operating point of the engine turning it. For 4-stroke gasoline engines of typical design this will be around 2500 RPM at which the engine is producing roughly a half horsepower per cubic inch of displacement. You then pick the maximum allowable prop diameter to keep the tips subsonic at 2500 RPM.
Next you assume the minimum number of blades for a start (2) and select the pitch angle at that diameter which will absorb the full output of the engine running at 2500 RPM. If two blades will not load the engine enough to hold it at 2500RPM at full throttle, then you add another blade and repeat the process.
For engines of less than about 180HP, the right answer is a two-bladed propeller of fixed pitch. For engines bigger than that, the right answer is generally a two- or three-bladed propeller with variable pitch.
One way to beat the sonic limit while getting more power out of the engine is to gear the engine down so it is spinning faster, and thereby generating more power, while still maintaining the prop tips subsonic. This also lets you further reduce the prop tip speed and go for a larger diameter prop if you have the ground clearance available, which will generally be more aerodynamically efficient than a smaller one- but running the engine faster shortens its service life and the cost to rebuild a geared engine has to include the cost to rebuild the gearbox itself.
For light aircraft in the homebuilt class or for ultralights, a less-expensive alternative to the geared engine is one with a rubber-belt reduction drive. This lets you use lighter and simpler 2-stroke engines which have a best power point of around 4500 RPM and still maintain the prop tips subsonic.

Thank you for the design process, the subsonic part was probably closest to what I was looking for. However, how do you determine where "the pitch angle will absorb the full output at 2500 RPM"? I do not exactly understand what you mean by that. Wouldn't the prop always absorb the power input, but it would just have different efficiencies at different RPM and thus convert it into varying amounts of useful power? When would two blades ever not be able to load the engine at a given power?

here is how it works. you bolt on a prop, start the engine, and advance the throttle.if the prop is pitched too deep, then at full throttle the engine is bogged down to, say, 2000RPM and is not putting out its full rated power. alternatively, if the prop is pitched too shallow, then when you advance the throttle you hit 2500RPM before the throttle is wide open and if you open it all the way, the engine goes overspeed and the prop tips go supersonic. The prop manufacturer has performance charts that spell all this out for you in advance.

For engines of less than about 180HP, the right answer is a two-bladed propeller of fixed pitch.

There a lot of planes with 180 HP engines (such as Lycoming O-360) with constant speed prop. Redline prop speed seems to be set to keep the tips from going subsonic, so prop length would also be a factor.  I don't know the math, but the prop total area has to be sized to absorb the engine power - so shorter length to stay out of the grass would need a wider blade.
I am guessing the 'right answer' was also based on a price point the plane manufacturer was trying to meet.  Fixed pitch cost is much less than the combination of constant speed blades, hub, different sized spinner, governor, control cable, manifold pressure gage, and of course an engine that can supply oil pressure for props controlled by oil pressure.


text:
Radar, radio, and Biplanes
In a book I'm writing they plan on having a biplane, similar to the RAF se5 but equipped with a radio, as their means of checking in on them. They are on an island where the enemy has WW2 era RADAR but they don't know that. My question is how would that really interact? Would the biplane be almost invisible because it's small, wooden and out at sea?

As this is a question about Aviation this is the right place.

WW-II-era radar technology actually spans a very wide range of technologies, from British 20 MHz Chain Home with dipole antennas to German 24 GHz prototypes with parabolic disks that surprised its designers by being unusable because nobody until then had figured out that atmospheric moisture dampens the signal at such frequencies.
An SE-5a type design has three items which will interact strongly with electromagnetic waves: The engine, the bracing wires and the control cables. Their radar response depends much on the radar frequency - that is why I started with the frequencies which were used back then. Wires act as dipole antennas and will resonate when radiated with a wavelength double their length (or corresponding harmonics). For a 5m bracing wire that would be 60 MHz, for example.
The size of the engine would require higher frequencies for best detection range. Today, the X-band (8-12 GHz) is most commonly used.
WW-II radars already used the Doppler frequency shift for determining the speed of discovered objects, most famously the British H2S ground mapping radar, but also the German FuG217 Neptun which, in combination with the Elfe-Gerät, would measure distance to the target and even automatically fire the guns when in range. While the Neptun range of radars operated at frequencies between 158 and 187 MHz, the most advanced developments of that era (like the FuG244 Bremen) already used the S-band (3.3 GHz).

@DanMašek Of course it should! Thank you!

To answer this question I am going to start on the plane itself. Being wooden does not mean the plane is invisible to radar because radar relies on bouncing a wavelength off of a object and then determining how far away the object is. Since wood is not a radar absorbent material it can still be detected by radar. Secondly in WW2 radars were used out at sea and that was one of their primary uses - so yes they work on objects that are out to sea. And thirdly in WW2 biplanes were still used and the newer 2-winged planes were very similar in size to the older biplanes so likely the radar would still pick up the biplane.

"Radars where used out at see and that was one of there ..." should that be "radars were used out at sea and that was one of their ..."? Homonyms are so difficult...

There are three things a material can do to radio: absorb, transmit, and reflect. Out of those, "reflect" is the one that wood does least. At lower frequencies it's mostly transparent, up around 5-6 GHz it starts to absorb significantly.

I wonder if there's anything written about the Mosquito (while a sophisticated monoplane, it was largely wooden) and its visibility on radar

@ChrisH: The Mosquito had enough metal to be thoroughly visible on radar.

I recall a fellow glider pilot saying years ago that he was surprised to find out that ATC was often able to detect glass-fiber gliders with radar. I don't think glass fiber composites are good radar reflectors, but gliders also contain metal parts such as control rods and cables.


text:
Why would a ram air parachute need less brake input to stall at a higher altitude?
I have some anecdotal evidence that a ram air sport parachute requires less toggle input to stall at higher altitudes. A pilot needed one wrap of the brake lines around the hands to quickly, definitively, and repeatedly stall above 6,000 feet or so, and two wraps below that.
The stall maneuver was:

fly in brakes for a few seconds to slow down
quickly lock out arms to maximum extension

Although I'm not discounting the possibility of this pilot being mistaken, I'm curious if this is expected behavior and what the physics behind it are (or might be) that are specific to parafoil type gliders. See my thoughts below on why a stall might be particularly complex here.
Update: Three different very experienced (in acro and/ or XC) paragliders just told me similar observations about higher altitude stalls: "lower brake pressure", "less brake travel/ range", "more dynamic", "similar to flying heavy or on a smaller glider".
What I found so far:

A stall is caused by too high AoA.

AoA, pitch, and IAS are expected to remain constant across altitudes for an unpowered parafoil in a stabilized glide.

TAS increases with altitude if IAS is constant

A rule of thumb to get TAS is to add 2% to the CAS/IAS for every 1000 feet of altitude.

This question about stall IAS varying with altitudes has an answer that seems to mostly apply to powered aircraft. It mentions Reynolds number for smaller aircraft. Would that really be significant here?

Random explanation of paraglider controls, which are essentially the same as for a ram air parachute.

More info in case it helps:
A parachute like this is about 200 square feet. The total system weight is around 200 lb. The wing itself weighs 10 lb including the lines/ risers, which are around 15 feet long. It attaches to the pilot's harness at only two points, one on each shoulder. The fabric forms an arch in flight around 20 feet wide tip to tip, 5 feet tall in the middle, and 10 feet at the deepest (tapered).
The indicated or sea level airspeed is around 40mph with no brake input and 20mph in deep brakes. The glide is between 2.5 to 3.0 for all speeds (so components are significant).
The stalls were practiced at random altitudes between 12,000 and 3,000 feet.
More thoughts:
This type of stall is a dynamic maneuver, and a ram air parachute is a soft wing with a CG far below the CL.
Here's my current understanding of what goes on before the stall fully develops:

pilot pulls down the brake lines
the wing changes shape, increasing camber and becoming less streamlined
the wing accelerates up and back as lift and drag increases
the pilot (ballast/ CG) has more momentum due to being much heavier than the wing and continues moving close to the original velocity for a while
line tension increases (increasing wing loading and therefore also stall speed?)
the wing pitches up due to the rolling moment caused by the CG moving forward relative to the CL
the stall starts to develop, reducing lift
at some point, I presume the stall causes enough loss of lift that the wing starts to deform (is this due to loss of line tension?), causing more loss of lift, etc... and allows the stall to progress even as the CG returns to below the CL, the wing pitches forward, and the lines slacken

Is it possible to analyze all this complexity to see which parts would be affected by altitude (e.g. air density or TAS) and whether that effect is significant?
Or am I overthinking this and there's some simple well known explanation?

It is said that "opening forces" are greater at higher altitudes.  I wonder if higher TAS has a greater effect when the bottom is "cupped" by pulling the brake lines.

@RobertDiGiovanni Good thought! That reminded me: Isn't the momentum of the pilot swinging forward a function of TAS and not IAS??

Yes.  But it may be more about the increased drag of the parachute with higher TAS.  Not sure exactly where the vertical CG is on that system (how much the pilot weighs vs how much the parachute weighs).  I am only recently studying these things, but the true airspeed and parachute chord seems to be right on the cusp of where Reynolds number becomes important.  So TAS of 100 km/hr vs 50 km/hour might have major effects.

If this were true, one would think that a similar effect would apply to paragliders, and would be well-known, because the paragliding community frequently holds clinics to practice various aerobatic maneuvers etc.  It might be worth your effort to find a paragliding on-line forum and simply ask if any similar variation of control effect with altitude has been observed, and then if you get a "yes" answer, ask what the cause is thought to be.   Would be interested to hear what you find--

@RobertDiGiovanni At these altitudes, the TAS would be more like 43 vs 48 from 3k to 9k feet assuming the 2% rule. (Keep in mind the horizonal component is significantly smaller, given 3 to 1 glide.) The entire wing including risers and lines weighs around 10lb.

@quietflyer Thanks, I asked it in a local paragliding chat, and an acro pilot said he definitely noticed a difference in stall point pressure and characteristics between two nearby practice locations at very different altitudes (within the same "weather system"), and that other acro pilots had agreed with him. He didn't notice a difference between the top and bottom of a specific practice run, but he attributes that to his typical mental state changes in a run . He noticed different but even bigger differences in rising or humid air. There doesn't seem to be an established explanation.

@quietflyer I updated the post with some quotes I got from the paragliders.

I was able to find a single research paper from Airborne Systems that reports something similar:

An increase in turn responsiveness, specifically turn rate for a given
stroke amount, has been observed on multiple planforms and sizes at
higher altitudes up to 25,000 ft. MSL.

The paper does not mention stalls/ pitch changes. However, the only difference between pitch changes and turns is two toggles down vs just one and the body swinging forward vs out to the side.
Summary of Relevant Section
The paper explains this as due to two factors (pages 4-6):

Mass ratio. The mass of the air inside the wing decreases as air density decreases, which moves the center of gravity down slightly and changes the moment of inertia.

Kinetic energy. Kinetic energy is proportional to the square of the velocity and thus increases greatly with altitude. See below for my current understanding of the effects of this.

Complexity
The mass ratio is pretty straightforward, but I had to hire an aerospace engineer and an astrophysics professor as tutors to try to get some formulas for the kinetic energy part.
We did find the formulas. It's a field called "flight dynamics", and there are even adaptions for parafoils that include the "apparent mass" of the air being "dragged along" by the canopy.
Unfortunately, the math is REALLY complicated. It would have taken my tutors more time than I wanted to pay for to simplify the equations down to just control changes with altitude.
If anyone wants to go through the trouble of modeling it, hit me up and I'll get you started.
Until that happens, I don't think this question is "truly" answered. However, it's enough to temporarily satisfy my curiosity. I figured I'd post the understanding I gained in the process, in case it helps someone.
My Current Understanding
I came up with the following explanation with the help of those tutors and a mentor who has built and designed a few dozen ultralight airplanes.
The pilot mass is 10 times that of the wing. When the wing wants to change direction due to increased angle of attack/ camber from the brake inputs, the pilot resists the change in motion for much longer, due to the 10x higher momentum. This causes the wing to rotate back and down due to the lines holding it back.
Momentum (which the paper does NOT mention) is what allows the pilot's body to keep moving forward (or out, for a turn) relative to the wing after a fast control input. It's proportional to velocity (not velocity squared). So the differences with altitude may or may not be significant, since it's proportional to all the other velocity-related changes (such as the radius of the resulting arc).
Since the wing is pitching up due to rotating back like this, lift and drag are increasing even more than what the initial control input would cause. Once the pilot body starts accelerating up and back, it "overshoots", swinging up relative to the canopy. This pitches the canopy up even more.
Kinetic energy (proportional to the square of the velocity) can be converted into potential energy (altitude). Thus, it determines how high the pilot's body swings up during the overshoot, and thus how far the canopy pitches back (or rolls, for a turn) at the maximum. This is the significant factor.
("Body swing" is an over-simplification. In reality, the body and wing swing/ glide/ rotate together, affecting each other dynamically.)
To get back to stalls: a stall is caused by too high angle of attack. A sudden pitch change increases the angle of attack. The kinetic energy determines the maximum pitch change caused by the body moving up relative to the canopy, and kinetic energy is much higher at high altitudes. Thus, the same control input at a higher altitude might stall the canopy even if it did not stall at a lower altitude.
Compared to other aircraft: Parafoils/ canopies are much more affected by this, since their center of lift is so far from the center of mass, resulting in a greater pitching/ turning moment when the body swings up. (Similar to a long prybar being more effective vs a short one.) This would explain why it's not a well known effect in general aviation/ fixed wing gliders.

This may have to do with "reduced" flare abilities at higher density altitudes.
Let's look at it this way: a lighter aircraft worries about excessive G's with full pitch application, whereas a heavier plane worries about stalling from too high an AoA.

the heavier plane cannot change course as fast as the lighter one

So it's rate of AoA increase will be higher.  The lighter plane's change in course "keeps up" with its change in pitch (literally rotation on the pitch axis relative to line of flight) better.
We then substitute higher and lower density altitude for heavier and lighter weight (or even larger and smaller wing), and the same effect is present.
As pointed out by the OP, Reynolds number may not change enough to be a major factor, but should not be discounted completely.
The thought that an object with a higher TAS, therefor a higher momentum, will be harder to turn does bear validity, in that it too will have a lower angle of departure from its course for a given perpendicular force applied because it's v, not its m, is higher.

Why can you substitute density altitude for weight? Since IAS is the same, wouldn't the air resistance be the same as well?

@Oleg I guess in the end it is momentum that makes the difference.  The amount of lift generated is close for same IAS (remember Reyniolds) but that same force is trying to turn a faster object.  mv.  Faster TAS makes has the same effect as more weight at the same TAS.

Interesting, another paraglider just used very similar language about his experience: "My impression is that it's easier to stall at high altitude. Things are definitely more dynamic. Similar to flying heavy or on a smaller glider."

If momentum and velocity are relative, and we don't have a ground reference, what are they relative to here? Is it really the velocity of the air molecules?

@Oleg required lift is constant, IAS is constant, but TAS isn't as altitude increases.  Think of how easy a paper airplane makes a loop.  As TAS grows, the same lifting force  try to turn a faster and faster object.  Based on IAS, the rate of pitch rotation does not change (around the pitch axis), but the aircraft is less and less able to curve its path to follow.

Do you happen to know what the formulas are that would apply here? I think I'm understanding your explanation intuitively, but I'm having a hard time relating it to my (old and dusty) college physics.

For example, does the distance between the CG and CL matter? Or the mass? Or does all that cancel out? (If they cancel out, wouldn't all aircraft be affected by this?)

@Oleg the formula is mv.  Simple as that, an object with a higher v is harder to turn.  This is why too strong a pitch input can't be followed (flight path), the pitch rotates past critical AoA before the flight path can change enough.  Think loop.  Why a little paper airplane can easily loop without stalling (but might have a harder time at 50,000 feet even though it's IAS is the same).